Several different types of chromogenic lactone compounds are described in U.S. Pat. No. 23,024, U.S. Pat. Nos. 3,491,112, 3,491,116, 3,509,173, 3,540,909, 3,540,911, 3,540,912, 3,736,337, 3,775,424, 3,853,869 and 4,020,068 and in Belgian Pat. No. 844,962.
U.S. Pat. No. 3,958,815 describes carbinol ether compounds containing indolizinyl groups.